1. Field of the Invention
The present invention relates to spectral sensitizing dyes. Photobleachable dyes particularly useful for the spectral sensitization of photoinitiators and photosolubilizers.
2. Background of the Art
Many different classes of compounds have natural sensitivities to various wavelengths of the electromagnetic spectrum. For example, various silver halide crystals are sensitive to ultraviolet radiation and short wavelengths of blue light, diazonium salts and diazo oxides are sensitive to ultraviolet radiation, iodonium and sulfonium salts are sensitive to ultraviolet radiation, and many other classes of photoinitiators such as s-triazines, bisimidazoles, etc. have natural sensitivities to narrow bands of radiation within the electromagnetic spectrum. Without the ability to expand these natural or inherent ranges of sensitivity, many imaging processes would not be useful.
Fortunately, most photosensitive materials can have their natural range of spectral sensitivity expanded. This can be done either by modification of the material itself or by the addition of a spectral sensitizing dye. The spectral sensitizing dyes generally are believed to function by absorbing radiation and transferring energy to the photosensitive material. Silver halide emulsions are conventionally sensitized to different portions of the spectrum (e.g., U.S. Pat. Nos. 3,936,308; 3,758,461) as are dry silver constructions (e.g., U.S. Pat. Nos. 4,476,220; 4,461,828), dye bleach and leuco dye oxidation systems (e.g., U.S. Pat. Nos. 4,460,677; 4,386,154; 4,373,020; 4,343,891) and positive and negative resist systems (e.g., U.S. Pat. Nos. 4,476,215; 4,571,374; 4,571,373).
Spectral sensitizers may often have different performance requirements depending upon the use of the final product. In black-and-white negative acting graphic arts film where unexposed areas are removed to form the final image, the sensitizing dye may remain in the final image where it is essentially overwhelmed by the density of the black image. In printing or proofing environments, where exact color renditions are essential and a range of color densities is necessary, the presence of residual colored sensitizing dye can adversely effect the quality of the image. In wet processed silver halide systems, it is often desirable to use alkaline solution bleachable sensitizing dyes. In thermally developed imaging systems it is desirable to use thermally bleachable sensitizing dyes. In these and other imaging systems, photobleachable dyes are highly desired.
U.S. Pat. No. 3,729,313 describes various classes of spectral sensitizing dyes useful in the sensitization of diaryliodonium light-sensitive compounds. The specific classes of dyes described there are useful sensitizers, but tend to be difficult to photobleach and the products of the bleach process are often themselves colored. Furthermore, the dyes tend to be migratory which can lead to speed variations and unpredictable staining problems.